This sounds like two issues. First, the soil that you are using may not be "heavy" enough, or may contain too much perlite or other inert media that helps fluff up the soil. Try a heavier soil mix. This can help some.

The next issue at hand will be that there may not be enough drainage in the container. If there is not enough means by which to let out water, then this can cause the bulbs to sink as well. Try drilling more holes, or finding a container with better drainage.

This is an unfortunate situation, but it can be easily corrected. Container gardening is definitely a trial and error process.